When radiation rays such as X-rays and gamma rays pass through a substance, absorption and scattering differ depending on a kind and a shape of the substance. It is possible to grasp a damaged state, a change, a filling state, and so on of the substance by recording the difference in the absorption and the scattering as, for example, photographs, video, digital files, and the like. Measurement of the absorption and scattering of X-rays is generally used as a method examining a state of an inside of a human body such as an X-ray photograph in a case of X-rays. This method of measuring a state of an inside without destroying a substance or a sample to be measured is called radiography or a nondestructive radiation photographing method.
In medical photographing using X-rays, primary X-rays are radially emitted from a focal point of an X-ray source, to be irradiated on a specimen. A part of the primary X-rays is absorbed by the specimen, and the remainder is attenuated as it is without changing an angle to pass through the specimen, and recorded by an image-receiving body. Meanwhile, when the primary X-rays are irradiated on the specimen, X-rays scatter in addition to being absorbed depending on the substance, and secondary X-rays, tertiary X-rays, and so on being scattered rays head for the image-receiving body while changing an angle from the primary X-rays.
When a transmission image of the specimen is to be obtained under this state, the secondary X-rays, the tertiary X-rays, and so on in addition to the primary X-rays are recorded on the image-receiving body. Accordingly, a clear transmission image cannot be obtained because a transmission image obtained by the scattered X-rays such as the secondary X-rays and the tertiary X-rays is overlapped with a transmission image obtained by the primary X-rays.
In this context, a grid is normally disposed between the specimen and the image-receiving body to obtain a clear transmission image by removing the scattered X-rays such as the secondary X-rays and the tertiary X-rays.
In a grid, a spacer part whose X-ray absorptance is low and absorption foil whose X-ray absorptance is high are arranged in a direction approximately in parallel to an irradiation direction of the primary X-rays, and they are layered in an approximately perpendicular direction to the irradiation direction. For example, fiber, resin, chip, or aluminum (Al) are used as the spacer, and foil containing a heavy element such as lead foil is used as the foil. As a result, the scattered X-rays such as the secondary X-rays and the tertiary X-rays having different angles from the primary X-rays are absorbed by the lead foil of the grid to be removed.
There are grids such as a focused grid where an angle of the grid is aligned with an angle of the primary X-rays in accordance with a distance from the focal point of the X-ray source to the image-receiving body, a parallel grid assuming that the primary X-rays are irradiated in parallel, and a tapered grid where heights of the lead foil at a center and at an outer side are different. Standards of the grids are described in JIS Z 4910:2015 as a guide.
There is also known a method to obtain a transmission image of a specimen by using neutrons as same as X-rays. This method is called neutron radiography, neutron imaging, or the like, and has been vigorously used in fields of fuel cell and engine containing hydrogen and hydrogen atoms in metal where water, resin, oil, alcohol, and so on are contained, and hydrogen storage which are almost impossible to photograph by conventional X-ray or gamma-ray radiography. This is because a scattering reaction of neutrons with hydrogen or the like having approximately the same mass is remarkable, and neutron has high sensitivity with water, plastic, and so on each containing hydrogen. These methods are suitable for imaging of specific neutron absorption materials such as gadolinium (Gd), cadmium (Cd), or boron (B).
However, there is a problem also in the case when the transmission image of the specimen is obtained by using neutrons as stated above that a clear transmission image cannot be obtained because scattering of neutrons occurs as same as the case of X-rays, and an image formed by the scattered neutrons overlaps with the transmission image. In a case of neutrons, unlike X-rays, a reaction with a constituent element of the specimen differs depending on energy of neutrons, and secondarily generated neutrons (scattered neutrons) also differ.
In a neutron radiography using a nuclear reactor as a neutron source, main components of used neutrons are thermal neutrons, and a main component of energy distribution thereof is 0.025 eV or less. However, there is a case when a very small amount of components of epithermal neutrons (EN) and fast neutrons (FN) which have higher energy than the thermal neutrons (TN) is contained also in the case of the nuclear reactor. When an accelerator is used as the neutron source, neutrons are widely distributed up to higher energy.
The fast neutrons react with hydrogen to be converted into the thermal neutrons. Accordingly, when the transmission image of the specimen is obtained by using the neutrons, the thermal neutrons are newly generated from the specimen different from the case using X-rays, an image formed by the thermal neutrons overlaps with a transmission image to be obtained by original thermal neutrons, resulting in that a clear transmission image cannot be obtained.